This invention relates generally to thin film batteries, and more particularly to thin film, rechargeable lithium ion batteries.
Conventional, canister type batteries today includes toxic materials such as cadmium, mercury, lead and acid electrolytes. These chemicals are presently facing governmental regulations or bans as manufacturing materials, thus limiting their use as battery components. Another limitation associated with them is that the amount of energy stored and delivered by these batteries is directly related to their size and weight. Large batteries, such as those found in automobiles, produce large amounts of current but have very low energy densities (Watts hours per liter) and specific energies (Watt hours per gram). As such, they require lengthy recharge times which render them impractical for many uses.
To address the need for higher energy densities and specific energies, the battery industry has been moving towards lithium based batteries. The major focus of the battery industry has been on liquid and polymer electrolyte systems. However, these systems have inherent safety problems because of the volatile nature of the electrolyte solvents. These types of batteries have a relatively high ratio of inert material components, such as the current collector, separator, and substrate, relative to the active energy storage materials used for the anode and cathode. In addition, their relatively high internal impedance results in low rate capability (watts/kilogram) which renders them insufficient for many applications.
Thin film lithium batteries have been produced which have a stacked configuration of films commencing with an inert ceramic substrate upon which a cathode current collector and cathode is mounted. A solid state electrolyte is deposited upon the cathode, an anode in turn deposited upon the electrolyte, and an anode current collector mounted upon the anode. Typically, a protective coating is applied over the entire cell. Lithium batteries of this type are describe in detail in U.S. Pat. Nos. 5,569,520 and 5,597,660, the disclosures of which are specifically incorporated herein. The lithiated cathode material of these batteries have a (003) alignment of the lithium cells, as shown in FIG. 1, which creates a high internal cell resistance resulting in large capacity losses.
Recently, it has been discovered that the annealing of lithiated cathode materials on a substrate under proper conditions results in batteries having significantly enhanced performances, for the annealing causes the lithiated material to crystallize. This crystallized material has a hexagonal layered structure in which alternating planes containing Li and Co ions are separated by close packed oxygen layers. It has been discovered that LiCoO2 films deposited onto an alumina substrate by magnetron sputtering and crystallized by annealing at 700xc2x0 C. exhibit a high degree of preferred orientation or texturing with the layers of the oxygen, cobalt and lithium are oriented generally normal to the substrate, i.e. the (101) plane as shown in FIG. 2. This orientation is preferred as it provides for high lithium ion diffusion through the cathode since the lithium planes are aligned parallel to the direction of current flow. It is believed that the preferred orientation is formed because the extreme heating during annealing creates a large volume strain energy oriented generally parallel to the underlying rigid substrate surface. As the crystals form they naturally grow in the direction of the least energy strain, as such the annealing process and its resulting volume strain energy promotes crystal growth in a direction generally normal to the underlying substrate surface, which also is the preferred orientation for ion diffusion through the crystal.
However, the limitations of these batteries have been the thickness and weight of their substrates upon which the layers of active material are laid upon. Because of the size of the substrate, these batteries have not been competitive with other formulations in terms of energy density and specific energy. High energy density cells have not been successfully constructed. The supporting substrates have been made of relatively thick sheets of alumina, sapphire, silica glass, and various other types of ceramic material. The substrate of these batteries typically constitute more than 95% of the total weight and an even larger percentage of the volume, thus only a small amount of the battery weight and volume is attributed to the active materials. This ratio of active material to the overall weight and volume of the battery limits their use. Furthermore, ceramic substrates and the like are generally inflexible. As such, these ceramic substrates can not be utilized to form batteries which are placed in a position where they may be bent, such as in xe2x80x9csmart cardxe2x80x9d applications.
Based on the prior art (as taught in Characterization of Thin-Film Rechargeable Lithium Batteries With Lithium Cobalt Oxide Cathodes, in the Journal of The Electrochemical Society, Vol. 143, No 10, by B. Wang, J. B. Bates, F. X. Hart, B. C. Sales, R. A. Zuhr and J. D. Robertson), liCoO2 annealed at a temperature below 600xc2x0 C. has no significant change in the microstructure, and thus the lithium orientation remains amorphous. This amorphous state restricts lithium ion diffusion through the layers of oxygen and cobalt, and therefore creates a high internal cell resistance resulting in large capacity losses.
Hence, in order to anneal the lithiated cathode material to the most efficient orientation it was believed that the cathode had to be bonded to a rigid substrate and heated to nearly 700xc2x0 C. for an extended period of time. Because of such extreme heating, it was believed that only certain metals with high melting points could be used as the cathode current collector. A problem associated with these metals has been their inability to bond with the substrate material, as these metals xe2x80x9cde-wetxe2x80x9d thereby forming small pools upon the substrate surface. As such, cathode current collectors have been made of cobalt overlaid with a layer of gold or platinum. During the annealing process the cobalt becomes a transition ion which is pulled through the gold or platinum and into the cathode material, thereby leaving the gold or platinum layer as the current collector.
It was believed that lightweight, low melting point metals and polymers could not survive the annealing process and therefore could not be used as a substrate in thin film lithium batteries having crystallized cathodes. This was a common belief even though such other materials would be chemically compatible for use with lithium cathodes. In addition, because of the high shrinking coefficient of polymers it was very difficult to use polymer in the construction of thin film batteries, as shrinkage causes a separation between the polymer and the overlying battery components.
It thus is seen that a need remains for a high performance rechargeable, thin film lithium battery which is smaller and lighter than those of the prior art. Accordingly, it is to the provision of such that the present invention is primarily directed.
In a preferred form of the invention, a thin film lithium battery comprises a polyimide support substrate, a cathode current collector mounted upon the polyimide support substrate, a crystallized lithium intercalation compound cathode coupled to the cathode current collector, an electrolyte deposited upon the lithium intercalation compound cathode, an anode deposited upon the electrolyte, and an anode current collector coupled to the anode.